cc presubmit: Add "master." prefix for the blink tryserver.

R=tandrii@chromium.org
BUG=617627
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2147793003
Cr-Commit-Position: refs/heads/master@{#405293}

cc/PRESUBMIT.py

@@ -338,7 +338,7 @@ def PostUploadHook(cl, change, output_api):
     return []
 
   bots = [
-    'tryserver.blink:linux_blink_rel',
+    'master.tryserver.blink:linux_blink_rel',
   ]
 
   results = []